Stereophonic phonograph



J. H. RlGGs ETAL 3,077,519

STEREOPHONIC PHONOGRAPH 4 Sheets-Sheet :l

Feb. 12, 1963 Filed NOV. 17, 1959 Feb. 12, 1963 J. H. Rises ETAL 3,077,519

STEREOPHONIC PHONOGRAPH Filed Nov. 17, 1959 4 Sheets-Sheet 2 f4 4H y. C//AAM/f/J g3 CHAN/VEL B Feb. 12, 1963 v J. H. RlGGs ETAL sTEREoPHoNIc PHONOGRAPH 4 Sheets-Sheet 3 will. "JET- n .All

llll 1 Feb- 12, 1963 J. H. RIGGs ETAL 3,077,519

STEREOPHONIC PHONOGRAPH Filed Nov. 17, 1959 4 Sheets-Sheet 4 :jj ZB'y 100 amm/EL A n /Jf y ,70g 54 J IN V EN TOR` United Stat The present invention relates to automatic coin operated phonographs or -recording playing machines, and is concerned particularly with problems involved in the control of such machines which are designed to play stereophonic records.

Automatic coin operated machines of this character are commonly installe-d in public yor semi-public places to play a selection of records in any selected order. lt is eX- tremely desirable that the volume or loudness of the sonic or acoustical output of an automatic phonograph of thisv character be maintained within an optimum range which is most acceptable in the environment in which the machine is installed. On one hand, a machine must have a sonic output which is suiciently loud to permit all patrons of the machine to enjoy the music. On the other hand, the output should not be so loud as to be offensive or irritating to others who may be present. Since such machines operate unattended, a problem arises in maintaining the acoustical output of individual machines within the optimum range of loudness for each specilic installation.

This problem arises from the fact that individua-l records played -on such machines have different degrees of inherent loudness. Thus, one record when played in an uncompensated machine may produce a sonic output volume greatly exceeding that of another record played under similar conditions. Moreover, the difference in the inherent loudness of individual records may exceed the optimum range of loudness for a machine in any speciiic location.

To maintain the sound level of such automatic machines within the optimum limits desired, it has been the practice to equip the machines with means which automatically compensates for dilerent degrees in the inherent loudness of the records played. ln general, the compensating means used for this purpose attenuates the output of the machines to a degree which is inversely proportional t-o the inherent loudness of the individual records. In this manner, relatively quiet or soft records are played with suiiicient amplication to be heard and enjoyed while the ampliiication of the playing of inherently louder records is reduced to remain within the acceptable sound range.-

The expedients previously used for this purpose have been acceptable in record playing machines designed to play monaural records. Here, the matter of compensating for different degrees of inherent loudness in successive records is simplified by the fact that only one sound signal is produced and amplied through a single amplification channel.

As will be presently explained in greater detail, the control means used for compensating for different degrees of record loudness in machines designed play monaural records is inadequate for handling the problems of volume or loudness control in machines designed to play stereophonic records, in which two sound signals are produced and separately amplified through separate amplification channels that energize a stereophonic speaker system.

One object lof the invention is to provide a new and improved stereophonic phonograph which will automatically compensate for the degree of inherent loudness in individual records played, while at the same time providing a fully eiective stereophonic reproduction of the recorded sounds.

arent A further object is to provide a new and improved multiple channel audio sound system which twill simultaneously and independently amplify a plurality of independent electrical sound signals in a manner which automatically compensates for the over-al1 volume or loudness level of the sound signals, While at the same time preserving the mutual independence yof the signals.

A further object is to provide a new and improved stereophonic phonograph which transforms stereophonic electrical sound signals into stereophonic sound without distortion and with undiminished stereophonic acoustical effectiveness, While at the same time automatically compensating for the degree of inherent loudness Iof the record being played through the use of relatively simple and dependable electrical amplifying and control means.

Other objects and advantages will appear from the tollowing description of the exemplary embodiment of the invention illustrated in the drawings, in which:

FIGURE l is a diagrammatic illustration of a stereophonic phonograph embodying the invention;

FlG. 2 is a diagrammatic illustration of a portion of the electronic circuit used in the phonograph of FIG. l;

FlG. 2A is a continuation of the circuit diagram of FIG. 2 which adjoins the right-hand edge of FIG. 2; and

FIG. 2B is a further continuation of the circuit diagram of FIG. 2. and adjoins the right-hand edge of FIG. 2A.

Having reference to the drawings in greater detail, the stereophonic phonograph forming the exemplary embodiment of the invention is illustrated schematically in FlG. l and identified by the number l0. As depicted schematically in IFIG. l, the phonograph 10 comprises a turntable 12 which supports and rotates a stereophonic record 14 in engagement with a stereophonic pickup 16. The machine is equipped with conventional automatic record changing devices and conventional coin controls well known in the art and requiring no specific description here. It is suicient to note that the machine operates to play a selection of records in any selected order, the successive records being automatically placed on the turntable 12, or the equivalent, and rotated in engagement with the pickup 16 for playing.

lt will be understood that stereophonic records are cut to have in eilect two sound tracks or sound recording surfaces which physically recorld two sound signals which are independent of each other. The pickup 16 itself is of a conventional construction and engages the two sound recording surfaces of a record 14 simultaneously, and produces two separate and independent electrical sound signals at two separate outputs 2i), 22 of the pickup.

The electrical sound signals supplied to the pickup out puts 20, 22 during playing of a record 14 are amplified independently of each other through two separate electronic amplifying channels illustrated diagrammatically in FIG. l, and identied respectively with the legends Channel A and Channel B. The outputs of the channels A and B are connected respectively to stereophonic speakers 24, 26 which are normally spaced from each other to produce the desired stereophonic sound effects.

Stereophonic records, like monaural records, diler from each other with regard to the degree of inherent loudness in the inidividual records. Hence, it is desirable that the eiiective output volume or `acoustical loudness of the phonograph 10 be controlled to compen-sate for the degree of inherent loudness of the stereophonic records being played to maintain the output volume of the instrument within the loptimum range desired in the environ- -ment in which the instrument is installed. Since such phonographs operate unattended any means used to compensate for the inherent loudness of successive records should be automatic in its operation.

The matter of providing in a stereophonic phonograph automatic -compensation for the inherent loudness of individual records is immensely complicated by the necessity for maintaining the full stereophonic effectiveness of the sound reproducing system.

In a stereoplionic phonograph of the character described, -the volumetric level or effective loudness of both sound signals, which are independent and separ-ately amplified, are affected by the inherent loudness of .the record being played. To maintain the over-all loudnes-s or inteiisi-tyV of the sound being produced within the desired optimum range, it is desirable to compensate for differences in the strength of the two sound signals from the pickup 1'6 due to diierent degrees of loudness of successive records being played.

Yet in providing compensation for different degrees of strength of the two signals from the pickup 16 due to the inherent loudness of different records, it is necessary to take into account that the electrical sound signals produced at the two pickup terminals 20, 22 not only vary in relation to each other, but it is essential to the achievement of the stereophonic sound effect desired that these signals at the two terminals vary independently of each other. Otherwise, the effectiveness of the stereophonic reproduction of the recorded sound would be greatly diminished or destroyed.

Thus, the use of independent loudness compensating means -in each of the sound `channels A and B to compensate or the .strength or -volume of the electric sound signals in the respective channels would greatly diminish the desired stereophonic acoustical effect. For example, at a particular period in the playing of a stereophonic record 14 a relatively strong -signal may be supplied at the pickup terminal 20 and amplified through the channel A while only a much weaker signal is supplied at the pickup terminal 22 and amplified through the channel B. The result is to produce a relatively high intensity of sound from the speaker 24 with a much lower 1nten sity of sound coming from the speaker 26. This is precisely the efiect desired inv stereophonic reproduction'of sound. However, with automatic volume compensation being independently applied to each of the channels A and B in the manner described, the amplification of the stronger signal applied -to the channel A would be suppressed in relation to the amplification of the weaker signal supplied through the channelfB with a consequent loss in stereophonic effectiveness.

As a practical matter, it is not feasible to compensate the overall volumetric acoustical output from `both speakers 24, 26 in accordance with the composite strength of both of the two signals being amplified. This expedient tends to destroy the independent operation of the two lamplifying channels A and B with cross modulation, distortion `or other interference with independent operation of the two amplifying channels.

Y The matter of controlling the output volume of both sound channels of a stereophonic phonograph in accordance with the strength of the signals in one of the channels is complicated by inherent characteristics of stereophonic recordings. Thus, in a recording .of an orchestration, for example, -there may be protracted periods in which `a very strong signal is supplied to one channel when only a very weak signal is supplied to the other ch-annel. Assuming the volumetric output of bot-h channels were compensated in Vaccordance with the signal. supplied to the channel which, in the assumed example, is supplied over a protracted period with the weaker signal, the stronger signal supplied to the channel not used as a control would be Y excessively amplified to produce excessive loudness in the playing'of the recording.

In accordance with the invention, the improved stereophonic phonograph is automatically controlled through the use of relatively simple and dependable means to compensate .for different degrees of inherent loudness in different records beingplayed, while at the same time pre- 4 serving .the full stereophonic effectiveness of the acoustical reproduction of lthe stereophonic sound signals.

Thus, with reference to the exemplary embodiment of the invention depicted in the drawings, the pickup 16, yFIGS. l and 2, is connected through a plug 28 and a socket 30 with the independent pickup output terminals 20, 22. As previously explained, the pickup 16 supplies two independent electrical sound signals or voltages to the respective terminals 20, 22.

The pickup terminals 20, 22 are connected through jacks 32, 34 with two independent volt-age amplifiers 36, 38 in the respective 'amplifying channels A and B. The voltage amplifiers 36, 38 include, respectively, two triodes 40, 42, together with suitable coacting resistors and capacitors interconnected with the tubes, as illustrated in FIG. 2. Plate voltage is supplied to the tubes 40, 42 through a conductor 44 leading from a power supply plug 46, FIG. 2B.

The previously mentioned voltage amplifiers 36, 38 function as first stage amplifiers in the respective channels A and B, as intimated. The output of the amplifier 36 is connected through a fixed lresistance 48 with the input of a second stage amplifier 50 in the channel A. The resistance 4S is made up of a plurality of resistors 52 connected together as shown in FIG. 2.

Similarly, the output of the first stage amplifier 38 in the channel B is connected through a fixed resistance 54, formed by a plurality of interconnected resistors 56 with the input of a second stage amplifier 58 in channel B.

As will presently appear, the resistaiices 48 and 54 connected in series between the first and second amplification stages in the respective channels A and B are used in automatically controlling the out-put volume of the respective channels.

Amplification is accomplished in the respective second stage amplifiers 50, 58 by cathode following triodes 60, 62.

The cathode of the tube 60 in the amplifier 50 is con nected through a conductor 64, FIG. 2, and a socket 66 with a loudness control 68 which is adjusted by a manual control 70. The output of the loudness control 68 is connected back through the socket 66 with a conductor 72 leading to the input of a third stage amplifier 74 in channel A. The amplifier 74 includes a triode 76.

In a similar manner, the output of .the cathode follower amplifier 58 is connected through a conductor 78, the socket 66, a loudness control ganged with the loudness control 68, the socket 66, and a conductor 82 with the input of a third stage amplifier 84 in channel B. The amplifier 84 comprises a .triode 86.

Output conductors 88, from the third stage amplifiers 74, 84 in the respective channels A and B are connected through tone control circuits 92, 94 of a generally conventional character, FIG. 2A, with conductors 96, 98 leading to separate phase inverters 100, 1-02 in the respective channels A and B, which drive independent push-pull power amplifiers 104, 106 in the respective channels, see FIG. 2B.

The power amplifier 104 in channel A is coupled through a transformer 108 with a loud speaker 24 which generates sound corresponding to the signals supplied at the pickup terminal 20.

In like manner, the output of the amplifier 106 is coupled through a transformer 112 to the loud speaker 26 which produces sound corresponding to the signals supplied by the. pickup 16 to the terminal 22.

The operating power for the phase inverters 100, v102 and for the push-pull amplifiers 104, 106 is supplied from the previously mentioned power plug `46 through power distributing conductors 114, FIG. 2B.

The overall output volume of the two speakers 24, 26 is automatically controlled to compensate for the degree of inherent loudness of the record 14 being played, in a manner which preserves the independent operation of the two channels A and B, while at the same time maintaining the full stereophonic effect of the reproduction ofthe stereophonically recorded sound.

' The two electric sound signals amplified in the respective electronic amplifying channels A and B are attenuated independently of each other by the controlled introduction of voltage drops across the resistances 48 and 56 interposed in the respective amplifying channels, as described.

For this purpose the output sides of the resistances 48, 56 are connected to ground through variable resistance triodes 11-6 and 118, as shown in FIG. 2. The admittance of the two tubes 116, 11S is controlled by a control voltage applied to the grids of both tubes through a conductor 120 leading from a time delay, averaging or integrating circuit 122, which is designed to produce an output voltage corresponding to the inherent loudness of the record being played.

The two variable resistance tubes 116, 113 shunt the respective resistances 48, 54 to ground and draw current through the resistances, and hence induce voltage drops across the respective resistances which are proportional to the admittances of the respective tubes 116, 118.

As will presently appear, the effect of this is to attenuate the electric -sound signals at the outputs of the respective lirst stage ampliiiers 36, 38 to a degree proportional to the admittances of the compensating discharge tubes 116, 118.

The previously mentioned circuit 122 used to supply an admittance controlling voltage to the compensating tubes 116, 118 is designed to avoid diminution of the stereophonic sound reproducing capabilities of the instrument, while at the same time providing a volume cornpensating control voltage which conforms to the inherent loudness of the record being played. To avoid interference with the operation of the channels with each other, the circuit 122 is connected to sense the volumetric or loudness level of the electric sound signals passing through only one of the channels, in this instance channel B. Moreover, the circuit is designed to respond to the inherent loudness of the record being played while at the same time avoiding interference with the desired ability of the two amplifying channels to operate over substantial periods of time at radically dilierent sound or loudness levels in order to properly reproduce stereophonic recordings. Thus, in accordance with the invention the volumetric output of both channels A and B is automatically compensated in accordance with the average or continuing general loudness or volumetric level of the signals supplied to channel B during the playing of an individual record. The volumetric level of the signals supplied to either channel, in this instance channel B, when averaged over an extended period of time is an accurate measure of the true loudness level of the recording in each of the channels of a typical record. The circuit 12.2 not only produces an output voltage which corresponds to the average strength of the sound signals in the channel B over an extended period of time, and hence to the inherent loudness of the record being played, but at the same time allows the channel A to amplify a strong signal to the optimum degree `desired over a rather extensive period of time While the channel B is supplied with only a much weaker signal. Similarly, the circuit 122 provides proper amplication of a weak signal supplied to channel A while a much stronger signal is being amplified over a rather extensive period of time in channel B,.thus maintaining the full stereophonic capability of the instrument unaiiected by the operation of the means used to control record loudness.

As will be described in greater detail, the record loudlness compensating circuit 122 operates to respond to the Joverall average value of the sound signals supplied to the channel B while at the same time allowing the 'channel B to operate continuously through -a period up to a maximum value of the order of ten seconds While supplied with a very strong sound signal without signiiicantly reducing the volumetric output of either channel. Hence, either channel A or channel B is free to amplify a strong signal over a ten second period while the other channel is amplifying a much weaker signal without interference with the record loudness control circuit. In the playing of stereophonic records which utilize both channels to obtain the desired stereophonic effect the duration of any period during which the signal supplied to one channel greatly and continuously exceeds the strength of the signal supplied to the other channel will not exceed ten seconds, the period in which the circuit 122 allows the channel B to amplify weak signals without increasing the amplication in channel AI Having reference to the drawings, it will be noted with reference to FIG. 2 that the record loudness compensating circuit 122 comprises an input amplifier formed by a triode 124V the grid of which is connected through a blocking capacitor 126 with the output of the iirst stage arnplifier 38 in channel B. Plate voltage is supplied to the tube 124 through a conductor 128 leading from the power supply plug 46.

The output of the amplifying tube 124 is connected through a blocking capacitor 130 with a rectifying tube 132. The output side of the capacitor 130 and the input Side of the tube 132 are grounded through a resistor 134.

The output side of the rectifying tube 132 is connected through a resistor 136 with the conductor 120 which supplies a control voltage to the volume compensating tubes 116, 118. Operate ends of the resistor 136 are grounded through two capacitors 138, 140.

The resistor 136 and the capacitors 138, 140 have values which operate this circuit to provide an output voltage on the conductor which corresponds to an averaging of the signal voltage supplied to the circuit 122 from the channel B over a time interval which is of sufficient extent to avoid more than an unnoticeable change in the voltage on the conductor 120 due to a substantial change in the signal voltage supplied to the circuit 122 over a ten second period. By Way of example, the resistor 136 can have a resistance of 1.5 megohms when the capacitors 138 and 140 have capacities of one and two microfarads, respectively.

The circuitry used in implementing the invention in the manner described is relatively simple and inexpensive and provides great dependability in operation.

It will be appreciated that the invention is not necessarily limited to the specilic construction illustrated but includes variants and alternatives within the scope of the invention as deiined by the claims.

The invention is claimed as follows:

l. In a phonograph for playing stereophonic records and automatically compensating for the inherent loudness of individual records, the combination of two signal arnpliiiers, an electrical pickup unit having two separate electrical outputs connected to said respective amplifiers to control the latter, two signal attenuating resistors coupled to the outputs of said respective amplifiers, an averaging circuit coupled to the output of only one of said signal amplifiers ahead of the coacting resistor, said averaging circuit including circuit means for producing an instantaneous electrical output which is a function of the average signal voltage of said one signal amplier over an extended period of time, two electronic discharge valves connected to the outputs of said respective signal arnpliiiers through said respective resistors, two power ampliiiers having inputs coupled through said respective resistors with said respective signal amplifiers, electrical sound producing means connected to said respective power amplifiers and means coupling the output of said averaging circuit to both of said discharge valves to control the admittances of both valves at once in accordance with the output of said averaging circuit to produce a con- 7 tinuous modulation of the signal attenuating effect of both of said resistors simultaneously to compensate for prolonged changes in the average output of said one signal amplifier.

2. In a multiple channel sound system, the combination of multiple electronic amplifying channels, multiple sources of electric sound signals connected to the inputs of said respective channels, sound generating means connected to the outputs of said respective channels, an attenuator connected into each of said channels to modify the output thereof, an averaging circuit coupled to only one of said channels ahead of said attenuator therein, said averagingcircuit including means for producing an electrical output which is a function of the average strength of the Isignals applied to the input of the averaging circuit over an extended period of time, and means coupling the output of said averaging circuit to both of said attenuators to control the output levels of both of said channels to compensate in the outputs of Aboth channels simultaneously for prolonged changes in the average strength of the sound signals from the'sound signal source connected to said one channel.

3. In a stereophonic -sound system, the combination of two electronic amplifying channels, two sources of electric sound signals connected to the inputs of said respective amplifying channels, sound producing means connected to lthe outputs of said respective channels, a resistor coupled into each of said amplifying channels in series therewith, a time delay integrating circuit coupled to one of said channels ahead of said resistor therein, said time delay integrating circuit including means for integrating the signal voltage Vin said one channel ahead of said resistor therein over an extended period of time to produce an electrical output which is a function of the average of said signal voltage over said extended period of time, two electronic discharge valvesV connected to the output `side of said respect-ive resistors in said respective channels, and means connecting the output ofsaid time delay integrating circuit to both said discharge valves to control the admittance of both valves simultaneously in accordance with the output of said time delay integrating circuit to produce in said resistors in both channels simultaneous signal modulating elects which compensate in the outputs of said sound producing means of both channels for prolonged changes in average strength of sound signals in said one channel ahead of said vresistor therein.

in an electronic amplifying system for amplifying stereophonic sound signals, the combination of two electronic amplifying channels, two sources of electric sound signals connected to the inputs of said respective amplifying channels, each amplifying channel including a voltage amplifier and a power amplifier coupled in series through an attenuating resistor electrically interposed therebetween, a pair of variable admittance discharge devices connected to said respective channels -between the attenuating resistor and power amplifier in each channel, volume control circuit means coupled `to only one of said channels between the voltage amplifier and the attenuating resistor therein, -said volume control circuit means including means'for continuously accumulating'an yelectrical charge in accordance with the signal voltage at-the output-of the voltage amplifier in said one channel, said volume control circuit means including means for dissipating the charge from said charge accumulating means, said charge accumulating means and said charge'dissipating means being related to each other to provide a charge in said charge accumultaing means which is a function of the lsignal voltage at the output of said one channel voltage amplifier over an extended period of time, and means connecting said volume control circuit -to both said Velectronic discharge devices to control the admittance of both devices in accordance with the instantaneous charge accumulated in said charge accumulating'means to produce in said -resistors in both channels simultaneous signal attenuating effects which 8 compensate in the outputs of both channels for prolonged changes in the average strength of signals in said one channel ahead of said resistor therein.

5. In a volume compensated, multiple channel audiosound system, the combination of two electronic amplifying channels, two sources of electric sound signals connected to the inputs of said respective channels, two electric sound generators connected to the outputs of said respective channels, electric volume control means coupled to each of said channels to modify the output thereof, an averaging circuit coupled to only one of said channels ahead of the volume control means coacting therewith, means coupling the output of said averaging circuit to `both said volume control means to control both of said control means to modulate at once the output levels of both of said channels to compensate in the outputs of both channels for prolonged changes in the average strength of sound signals in said one channel ahead of the coacting volume control means, said averaging circuit including a series resistor and two capacitors connecting opposite ends of said resistor to ground, and said resistor and said capacitors having electrical values which produce at the output end of said resistor an electrical output which is a function of the average electrical signal applied to the input of the averaging circuit over an extended period of approximately ten seconds duration.

6. A phonograph for playing stereophonic records unattended with automatic compensation for differences in the inherent loudness level of different records, comprising, in combination, a stereophonic pickup unit, means for moving a stereophonic recording in association with said pickup unit to activate the latter, two separate amplifying channels connected to said stereophonic pickup unit to be controlled by the latter, two loudspeakers energized by said respective amplifying channels, two independent volurne control circuits coupled with said respective amplifying channels to control the outputs of the latter, a volume compensating circuit having an output connected in controlling relation to both said volume control circuits to impose volume control on both said channels, means coupling said volume compensating circuit to one of said amplifying channels ahead of said volume control circuit coacting therewith, and said volume compensating circuit including electrical integrating means for integrating over an extended period of time electrical signals supplied to said compensating circuit and providing an electrical output as applied to said volume control circuits which is a function of the average intensity over an extended period of time of the electrical signals supplied to the input of said volume compensating circuit and which serves through control of both of said control circuits at once by said volume compensating circuit to compensate in the loudness level of the loudspeakers of both channels for changes over said extended period of time in the average strength of sound signals in said one channel ahead of said control circuit therefor.

7. A stereophonic recording player for playing stereophonic recordings with automatic volume compensation for different degrees of inherent loudness of different recordings, said player comprising, in combination, a stereophonic pickup having two outputs for two electrical sound signals, meansV for moving a stereophonic recording in association with said pickup to activate the latter to produce stereophonic sound signals at the outputs thereof, two voltage amplifiers connected to the respective outputs of said pickup, two attenuating resistors connected to the outputs of said respective voltage amplifiers, two electronic discharge valves connecting said respective resistors to ground, a volume control amplifier having an input connected to the outputof one of said voltage amplifiers ahead of the coacting one of said resistors, a rectifier connected to the output of said volume control amplier, a volume control resistor connected between the output of said rectitier and both of said discharge valves to control the admittances of both valves simultaneously in accordance with the voltage supplied from said volume control resistor, two volume control capacitors connected between opposite ends of said volume control resistor and ground, said volume control resistor and said volume control capacitors having effective values related to each other to provide an integrated volume control Voltage which corresponds to an integrated averaging of the voltage output of said volume control amplifier over an extended period of time of approximately ten seconds, two power ampliers having inputs connected to respond to the outputs of said respective voltage amplifiers as modified by the flow of current through the attenuating resistors associated therewith so that the output levels of both power amplifiers are modulated simultaneously in accordance with said integrated volume control voltage to compensate for prolonged changes in the average strength of sound signals from the pickup output connected to said one voltage amplifier, and two loudspeakers energized by the outputs of said respective power amplifiers.

8. A stereophonic recording player for playing stereophonic recordings with automatic compensation for different degrees of inherent loudness of recordings played, said player comprising, in combination, a stereophonic pickup unit having two outputs for two electrical sound signals, means for moving a stereophonic recording in association with said pickup to activate the latter to produce stereophonc sound signals at said outputs thereof, means forming two separate amplifying channels connected to the respective outputs of said pickup unit, two signal attenuators coupled to said respective amplifying channels, a volume compensating amplifier having an input coupled to one of said channels, a rectiiier connected to the output of said volume compensating amplifier, a resistor connected to the output of said rectifier, two capacitors connected between opposite ends of said resistor and ground, said resistor and said capacitors having effective values related to each other to provide an integrated volume control voltage at the output end of said resistor which corresponds to an integrated averaging of the level of the input signal to said one amplifying channel over an extended period of time, electrical control means coupling said resistor in controlling relation to both said attenuators to operate the latter in unison for modifying the output of both said channels at once in accordance with the volume control voltage at the output end of said resistor to effect compensations in the output volumes of both said channels for prolonged changes in the average strength of sound signals supplied to said one channel from the corresponding pickup outlet, and two loudspeakers energized by the outputs of said respective amplifying channels.

9. A recording player for playing stereophonic recordings With automatic compensation for different degrees of inherent loudness of the recordings played, said player comprising, in combination, a stereophonic pickup, means for moving a stereophonic recording in association with said pickup to activate the latter, means forming two separate amplifying channels connected to said pickup to be controlled by the latter, two loudspeakers energized by said respective amplifying channels, two signals attenuators coupled with said respective amplifying channels to control the outputs of the latter, a volume compensating circuit having an output coupled to both said attenuators in controlling relation thereto to impose volume control on both of said channels simultaneously means coupling said volume compensating circuit to one of said amplifying channels, and said volume compensating circuit including electrical integrating means for integrating over an extended period of time electrical signals supplied to said compensating circuit and providing in controlling relation to both of said attenuators an electrical output which is a function of the average intensity over an extended period of time of the electrical signals moving through said one channel and which serves through control of both said attenuators to effect in the output levels of both said channels automatic compensation for prolonged changes in the average strength for sound signals supplied to said one amplifying channel from said pickup.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. IN A PHONOGRAPH FOR PLAYING STEREOPHONIC RECORDS AND AUTOMATICALLY COMPENSATING FOR THE INHERENT LOUDNESS OF INDIVIDUAL RECORDS, THE COMBINATION OF TWO SIGNAL AMPLIFIERS, AN ELECTRICAL PICK-UP UNIT HAVING TWO SEPARATE ELECTRICAL OUTPUTS CONNECTED TO SAID RESPECTIVE AMPLIFIERS TO CONTROL THE LATTER, TWO SIGNAL ATTENUATING RESISTORS COUPLED TO THE OUTPUTS OF SAID RESPECTIVE AMPLIFIERS, AN AVERAGING CIRCUIT COUPLED TO THE OUTPUT OF ONLY ONE OF SAID SIGNAL AMPLIFIERS AHEAD OF THE COACTING RESISTOR, SAID AVERAGING CIRCUIT INCLUDING CIRCUIT MEANS FOR PRODUCING AN INSTANTANEOUS ELECTRICAL OUTPUT WHICH IS A FUNCTION OF THE AVERAGE SIGNAL VOLTAGE OF SAID ONE SIGNAL AMPLIFIER OVER AN EXTENDED PERIOD OF TIME, TWO ELECTRONIC DISCHARGE VALVES CONNECTED TO THE OUTPUTS OF SAID RESPECTIVE SIGNAL AMPLIFIERS THROUGH SAID RESPECTIVE RESISTORS, TWO POWER AMPLIFIERS HAVING INPUTS COUPLED THROUGH SAID RESPECTIVE RESISTORS WITH SAID RESPECTIVE SIGNAL AMPLIFIERS, ELECTRICAL SOUND PRODUCING MEANS CONNECTED TO SAID RESPECTIVE POWER AMPLIFIERS AND MEANS COUPLING THE UTPUT OF SAID AVERAGING CIRCUIT TO BOTH OF SAID DISCHARGE VALVES TO CONTROL THE ADMITTANCES OF BOTH VALVES AT ONCE IN ACCORDANCE WITH THE OUTPUT OF SAID AVERAGING CIRCUIT TO PRODUCE A CONTINUOUS MODULATION OF THE SIGNAL ATTENUATING EFFECT OF BOTH OF SAID RESISTORS SIMULTANEOUSLY TO COMPENSATE FOR PROLONGED CHANGES IN THE AVERAGE OUTPUT OF SAID ONE SIGNAL AMPLIFIER. 